Published Unexamined Japanese Patent Application No. SHO 57-87344 is known and proposes an extrusion mixer comprising a rotor and a stator which are formed with semispherical cavities. The extrusion mixer (hereinafter referred to as "CTM") is so adapted that a fluid, while passing over the cavities, is subjected to a uniform blending action by being cut and sheared owing to the movement of the fluid from cavity to cavity and to the rotation of the rotor. Accordingly, the mixer exhibits remarkable blending and dispersing ability, for example, when preparing a compound from molten polyethylene and a carbon master batch composed similarly of polyethylene and carbon black dispersed therein in high concentration by uniformly blending the master batch with the melt, or when preparing a polymer blend from at least two kinds of molten polymers, which are different in flowability, by uniformly blending the melts together.
With the CTM, however, it is extremely difficult to obtain a homogeneous dispersion by blending a filler, such as carbon black or calcium carbonate, directly with polyolefin, because carbon black, calcium carbonate or like powder flows into the cavities before being fully kneaded into the polymer, filling up the cavities and rendering the machine inoperative for blending action. Further in order to knead agglomerates of particles into the polymer to obtain a uniform dispersion, the agglomerates must be broken down by shearing forces, but the shearing force acting on the fluid within the CTM is small and insufficient to break down the agglomerates of particles.
When mixing particles with a polymer to obtain a blend which is uniform on a microscale, the materials needs to be crushed or mashed by a shearing force, suitably with use of a mixer or kneader having such crushing or mashing action. It is well known that machines, such as Banbury mixers, in which the charge is crushed in a wedge-shaped space, are suitable for this purpose.